Apparatus for electrolytic treatment



April 9, 1946. J. B. NILES 2,398,110

APPARATUS FOR ELECTROLYTIC TREATMENT Filed Oct. 10, 1941 AT 1 5 a a 0 0Q T IT 1F l o 2 5 r T o a [ti 5 6 O O 0 Q I'IIIIIIIIII' Patented Apr. 9,1946 APPARATUS FOR ELECTROLYTIC TREATMEN'].

Joseph B.

Niles, Chicago,

11]., assignor to Thomas J. Dee & Lo., a corporation of IllinoisApplication October 10, 1941, Serial No. 414,509

Claims. (Cl. 204-56) This invention relates to an improved leadsilveralloy for use in connection with apparatus for electroplating,anodizing, or the like.

The primary object of the invention is to provide an improved alloy oflead and silver which will have long life in spite of being subjected toelectrolytes containing chromic acid, sulphuric acid, or other highlycorrosive liquids. The alloy is of particular use for making linings,heating coils, temperature indicators, and temperature regulatinginstruments employing fully immersed bulbs containing expansive liquidsor thermocouples. Bulbs to be protected when immersed in chromic acidsolutions may have an outer covering of the alloy of suitable thickness.

A further object of the invention is to provide an alloy which may beused as a cathode or anode in an electrolyte which in use will form anoxide of high electrical conductivity and will not seriously interferewith the maintenance of current densities necessary to efllcientelectrolytic operations.

Heretofore, it has been common practice to use a lead-antimony materialand it has been necessary to frequently clean the exposed surfaces.because the chromic acid solution quickly forms compounds such aslead-chromate.

In chromium plating work, because of the low electrical conductivity ofthe electrolyte, high amperages are ordinarily employed. This currentvalue may range from 115 amperes per square foot area of articles beingplated to 800 amperes per square foot, and there is produced anobjectionable amount of compounds on the anode surface. An analysis ofthese compounds shows them to be a mixture of compounds such as leaddioxide (PbOi), lead suboxide (PbzO), and lead chromate (PbCrOr). All ofthese compounds, because of their low electrical conductivity values,seriously interfere with the maintenance of current densities necessaryto continuous chromium deposition. Compounds that interfere with theflow oi the electrical current must be removed at frequent intervals torestore the anode surface to proper working efllciency. In commercialpractice, it is found necessary in some cases to remove the anodes forcleaning several times a day.

These difllculties have been overcome by the present invention throughthe use'of a lead-silver alloy. It was found that the compounds ofsilver formed in an electrolyte of chromic acid, when subjected to therequired current densities, show excellent electrical performance, andthat the amount of current drop was directly proportionate to the amountof silver combined with lead up to a percentage of about 8% of silverpresent. Anodes containing upwards of 8% of silver indicated nomeasurable current drop during a steady current flow for the duration ofan hour; this time being considerably in excess of that required in theconventional chromic acid solution used in the chromium plating ofarticles for decorative, or ornamental, or oxidation resistant purposes.I i

Silver-lead alloys containing more than 10% silver by weight and used asanodes in chromium plating work oilfered no appreciable advantage inthis respect. However, it was found that silverlead alloys used in thiswork and containing more than 12% silver by weight gave indications ofsilver migrating from the anode to the cathode under electrolysis, andit appeared as metallic silver codeposited with metallic chromium.Silver-lead alloys containing 12% or less silver produced no apparentsilver at the cathode under electrolysis.

In a series of tests, 6 lead-silver alloys were prepared, the firsthaving 4% silver, the second 6%, etc., up to 14%, inclusive. 6one-gallon aqueous solutions of chromic acid were made, each containing53 ounces of chromic acid and .53 ounces of 66 B. sulphuric acid. Intoeach of these solutions was placed one of the above mentioned alloys toserve as an anode. The surface area of each anode exposed to the chromicacid solution in all cases was 8 square inches. Also, into each of thesesolutions was placed a strip of chemically clean pure nickel, eachhaving an exposed area to the chromic acid solution of 8 square inches.The nickel, of course, served as a cathode in each case and a currentdensity of 18 amperes at 6 volts was then impressed on the surface ofeach lead-silver alloy anode and continued until an ampere meteremployed in the electrical circuit gave indications of current drop.

In observing the electrical performance of these various compositions oflead-silver anodes in this test, it was found that those alloys withless than 8% of silver gave indications of current drop or gradualfalling of! of current densities as the testing progressed.

An electrode containing 9 parts of lead to 1 part of silver showsthoroughly satisfactory results in a full size commercial installation.The conventional equipment is illustrated in the accompanying drawing,in which- Figure l is a plan view from above showing the-open top tankand the anode and cathode bus-bars; and Figure 2 is a sectional view,taken as indicated at line 2 of Figure 1.

In the equipment illustrated a suitable tank 3 is lined with thelead-silver alloy, as indicated at 4, so that the lining fully protectsthe outer shell. Cathode bus-bars 5 extend over the tank and aresupported in suitable insulating brackets 6. Also, the anode bus-bars Iextend over the tank and are supported in insulating brackets 8. Theanodes are formed of the lead-silver alloy and are provided at theirupper ends with conventional hooks III to enable them to be suspendedfrom the bus-bars. The temperature of the bath may be maintained bysuitable coils (not shown) in the electrolyte or by apparatus outside ofthe tank 3. Preferably, the lining of the anodes and other parts of theequipment that contact the electrolyte are made of the lead-silveralloy. It will be understood that the articles to be plated will besuspended from the bus-bar 5.

The lining made up of about 9 parts lead to 1 part of silver ispreferably about A; of an inch thick and the seams and joints renderedliquid tight.

Although the initial cost of this lining material is more than thecommon lead antimony lining, the new alloy will provide a substantialsaving because the tanks do not have to be taken out of operation forre-lining at such frequent intervals and the combined cost of the leadantimony alloy and labor for several re-linings is in excess of the costof one installation of the leadsilver lining.

It will be understood that the lead-silver alloy lining may be used withany suitable backing frame such as wood, metal, or the like. Ifpreferred, the alloy may be rolled onto a metal backing before beingassembled into a tank.

In the anodic treatment of aluminum, a process whereby a coating ofaluminum oxide is produced on the surface of aluminum by electrolysis,similar apparatus to that described above may be ised except that thealuminum forms the anode and the lead-silver alloy the cathode, whichmay be either separate electrodes or the lining of the tank.

Because of the higher current pressures used in this work volts). thesame dimculties are encountered when a lead-antimony alloy is used, andthese dimculties are overcome by the use of the lead-silver alloy. Theelectrolytes used for either the anodic treatment of the aluminum orchromium plating consist of the same oxidizing reagents, namely, chromicacid and sulphuric acid. By using a lead-silver alloy consisting of 9parts of lead to 1 part of silver as cathodes, either of the suspendedimmersed type or as the tank lining type, the same excellent electricalconductivity of the oxides of silver is utilized to advantage.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom for some modifications will be obvious to those skilled in theart.

I claim:

1. An electrode consisting essentially of 6% to 12% of silver and thebalance being substantially all lead and having on a surface thereof acoating of lead chromate and silver chromate having substantially thesame conductivityin a chromic acid plating bath as the underlying alloy,

2. An electrode consisting essentially of 8% to 12% of silver and thebalance being substantially all lead and having on a surface thereof acoating of lead chromate and silver chromate having substantially thesame conductivity in a chromic acid plating bath as the underlyingalloy.

3. An electrode as set forth in claim 1, in which the electrode is acathode.

4. The method which comprises passing an electric current through a bathcontaining chromic acid in which one of the electrodes exposed to thebath has an exposed surface of lead-silver alloy containing from 6% to12% silver,'whereby a surface coating of lead-silver chromate is formedwhich is substantially insoluble in the solution and which does notappreciably affect the resistance of the system. 7

5. The method as set forth in claim 4, in which the proportions of themetals are approximately 9 parts of lead to 1 part of silver.

JOSEPH B. NILES.

